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Exclusive production of pion and kaon meson pairs in two photon collisions at LEP
A. Heister, S. Schael, R. Barate, R. Bruneliere, I. de Bonis, D. Decamp, C.
Goy, S. Jezequel, J.P. Lees, F. Martin, et al.
To cite this version:
A. Heister, S. Schael, R. Barate, R. Bruneliere, I. de Bonis, et al.. Exclusive production of pion and kaon meson pairs in two photon collisions at LEP. Physics Letters B, Elsevier, 2003, 569, pp.140-150.
�in2p3-00014001�
CERN-EP/2003-030
20 May 2003
Exclusive Production of Pion and
Kaon Meson Pairs in Two Photon
Collisions at LEP
The ALEPH Collaboration
*
Abstract
Exclusiveproductionof andKmesonpairsintwophotoncollisionsismea-
suredwithALEPHdatacollectedbetween 1992and2000. Crosssectionsare
presented as a function of cos
and invariant mass, for jcos
j < 0:6 and
invariantmassesbetween2.0and6.0GeV =c 2
(2.25and4.0GeV =c 2
)forpions
(kaons). The shape of the distributions are found to be well described by
QCD predictions but the data have asignicantly higher normalisation.
Submitted to Physics Letters B
*
Seenextpageforthelistofauthors
A.Heister, S.Schael
PhysikalischesInstitutdasRWTH-Aachen,D-52056Aachen,Germany
R. Barate, R. Bruneliere, I. De Bonis, D. Decamp, C. Goy, S. Jezequel, J.-P. Lees,
F.Martin,E. Merle,M.-N.Minard,B.Pietrzyk, B.Trocme
Laboratoire de Physique des Particules (LAPP), IN 2
P 3
-CNRS, F-74019 Annecy-le-Vieux Cedex,
France
S.Bravo,M.P.Casado,M.Chmeissani,J.M.Crespo,E.Fernandez,M.Fernandez-Bosman,
Ll.Garrido, 15
M.Martinez,A.Pacheco,H.Ruiz
Institut de F
isica d'Altes Energies, Universitat Autonoma de Barcelona, E-08193 Bellaterra
(Barcelona),Spain 7
A. Colaleo, D. Creanza, N. De Filippis, M. de Palma, G. Iaselli, G. Maggi, M. Maggi,
S. Nuzzo, A. Ranieri, G. Raso, 24
F. Ruggieri, G. Selvaggi, L. Silvestris, P. Tempesta,
A.Tricomi, 3
G.Zito
Dipartimentodi Fisica,INFN SezionediBari,I-70126Bari,Italy
X. Huang, J. Lin, Q. Ouyang, T. Wang, Y. Xie, R. Xu, S. Xue, J. Zhang, L. Zhang,
W.Zhao
InstituteofHighEnergyPhysics,AcademiaSinica, Beijing,ThePeople'sRepublic ofChina 8
D.Abbaneo,T.Barklow, 26
O.Buchmuller, 26
M.Cattaneo,B.Clerbaux, 23
H.Drevermann,
R.W.Forty,M.Frank,F.Gianotti,J.B.Hansen,J.Harvey,D.E.Hutchcroft, 30
,P.Janot,
B. Jost, M. Kado, 2
P. Mato, A. Moutoussi, F. Ranjard, L. Rolandi, D. Schlatter,
G.Sguazzoni,F.Teubert,A. Valassi, I.Videau
EuropeanLaboratoryforParticlePhysics(CERN),CH-1211Geneva23,Switzerland
F.Badaud,S.Dessagne,A.Falvard, 20
D.Fayolle,P.Gay,J.Jousset,B.Michel,S.Monteil,
D. Pallin, J.M.Pascolo,P.Perret
Laboratoire de Physique Corpusculaire, Universite Blaise Pascal, IN 2
P 3
-CNRS, Clermont-Ferrand,
F-63177Aubiere,France
J.D.Hansen,J.R.Hansen,P.H. Hansen,A.C.Kraan,B.S.Nilsson
NielsBohrInstitute,2100Copenhagen, DK-Denmark 9
A.Kyriakis,C.Markou,E.Simopoulou,A.Vayaki,K.Zachariadou
NuclearResearchCenterDemokritos(NRCD), GR-15310Attiki,Greece
A.Blondel, 12
J.-C.Brient, F.Machefert,A.Rouge,H. Videau
LaoratoireLeprince-Ringuet,EcolePolytechnique,IN P -CNRS, F-91128PalaiseauCedex,France
V.Ciulli,E.Focardi,G.Parrini
Dipartimentodi Fisica,UniversitadiFirenze,INFN Sezionedi Firenze,I-50125Firenze,Italy
A. Antonelli,M. Antonelli,G.Bencivenni,F. Bossi,G. Capon,F. Cerutti, V. Chiarella,
P.Laurelli,G.Mannocchi, 5
G.P.Murtas,L. Passalacqua
LaboratoriNazionalidell'INFN(LNF-INFN),I-00044Frascati,Italy
J.Kennedy,J.G.Lynch,P.Negus,V.O'Shea,A.S.Thompson
DepartmentofPhysicsandAstronomy,UniversityofGlasgow,GlasgowG128QQ,UnitedKingdom 10
S.Wasserbaech
UtahValleyStateCollege,Orem,UT84058,U.S.A.
R. Cavanaugh, 4
S. Dhamotharan, 21
C. Geweniger, P. Hanke, V. Hepp, E.E. Kluge,
A.Putzer,H.Stenzel,K.Tittel,M.Wunsch 19
Kirchho-InstitutfurPhysik,Universitat Heidelberg, D-69120Heidelberg,Germany 16
R.Beuselinck,W.Cameron,G.Davies,P.J.Dornan,M.Girone, 1
R.D.Hill,N.Marinelli,
J.Nowell,S.A.Rutherford, J.K.Sedgbeer,J.C.Thompson, 14
R. White
DepartmentofPhysics,Imperial College,LondonSW7 2BZ,UnitedKingdom 10
V.M.Ghete,P. Girtler,E.Kneringer,D. Kuhn,G.Rudolph
Institut furExperimentalphysik,UniversitatInnsbruck,A-6020 Innsbruck,Austria 18
E. Bouhova-Thacker, C.K. Bowdery, D.P. Clarke, G. Ellis, A.J. Finch, F. Foster,
G.Hughes,R.W.L.Jones,M.R.Pearson,N.A.Robertson,M.Smizanska
DepartmentofPhysics,UniversityofLancaster,LancasterLA14YB,United Kingdom 10
O.vanderAa,C.Delaere, 28
G.Leibenguth, 31
V.Lemaitre 29
Institut de Physique Nucleaire, Departement de Physique, Universite Catholique de Louvain, 1348
Louvain-la-Neuve,Belgium
U. Blumenschein, F. Holldorfer, K. Jakobs, F. Kayser, K. Kleinknecht, A.-S. Muller,
B.Renk,H.-G.Sander,S.Schmeling,H. Wachsmuth,C.Zeitnitz,T.Ziegler
Institut furPhysik,UniversitatMainz,D-55099Mainz,Germany 16
A.Bonissent,P.Coyle,C.Curtil,A.Ealet,D.Fouchez,P. Payre,A.Tilquin
CentredePhysiquedesParticulesdeMarseille,UnivMediterranee,IN 2
P 3
-CNRS,F-13288Marseille,
France
F.Ragusa
Dipartimentodi Fisica,UniversitadiMilano eINFN SezionediMilano,I-20133Milano,Italy.
27
Max-Planck-InstitutfurPhysik,Werner-Heisenberg-Institut,D-80805Munchen,Germany 16
J.Boucrot,O.Callot,M.Davier,L.Duot,J.-F.Grivaz,Ph.Heusse,A. Jacholkowska, 6
L. Serin,J.-J.Veillet
Laboratoiredel'AccelerateurLineaire,UniversitedeParis-Sud,IN 2
P 3
-CNRS,F-91898OrsayCedex,
France
P. Azzurri, G. Bagliesi, T. Boccali, L. Foa, A. Giammanco, A. Giassi, F. Ligabue,
A. Messineo, F. Palla, G. Sanguinetti, A. Sciaba, P. Spagnolo R. Tenchini A. Venturi
P.G.Verdini
Dipartimento di Fisica dell'Universita, INFN Sezione di Pisa, e Scuola Normale Superiore, I-56010
Pisa,Italy
O.Awunor,G.A.Blair,G.Cowan,A.Garcia-Bellido,M.G.Green,L.T.Jones,T.Medcalf,
A.Misiejuk, J.A.Strong,P.Teixeira-Dias
DepartmentofPhysics,RoyalHolloway&BedfordNewCollege,UniversityofLondon,Egham,Surrey
TW20OEX,United Kingdom 10
R.W.Clit,T.R. Edgecock,P.R.Norton,I.R.Tomalin,J.J.Ward
ParticlePhysicsDept.,RutherfordAppletonLaboratory,Chilton,Didcot,OxonOX11OQX,United
Kingdom 10
B.Bloch-Devaux, D. Boumediene, P. Colas, B. Fabbro, E. Lancon, M.-C.Lemaire,
E.Locci,P.Perez,J.Rander,B.Tuchming,B.Vallage
CEA, DAPNIA/Service de Physique des Particules, CE-Saclay, F-91191 Gif-sur-Yvette Cedex,
France 17
A.M.Litke,G.Taylor
InstituteforParticlePhysics,UniversityofCaliforniaatSanta Cruz,SantaCruz,CA95064,USA 22
C.N.Booth,S. Cartwright,F.Combley, 25
P.N. Hodgson,M.Lehto,L.F. Thompson
DepartmentofPhysics,UniversityofSheÆeld,SheÆeldS37RH,United Kingdom 10
A.Bohrer,S. Brandt,C.Grupen,J.Hess,A. Ngac,G.Prange
FachbereichPhysik,UniversitatSiegen,D-57068Siegen,Germany 16
C.Borean,G.Giannini
Dipartimentodi Fisica,UniversitadiTriesteeINFNSezionedi Trieste,I-34127Trieste,Italy
H.He,J.Putz,J.Rothberg
ExperimentalElementaryParticlePhysics,UniversityofWashington,Seattle,WA98195U.S.A.
S.R. Armstrong, K. Berkelman, K. Cranmer, D.P.S. Ferguson, Y. Gao, 13
S. Gonzalez,
O.J. Hayes, H. Hu, S. Jin, J. Kile, P.A. McNamara III, J. Nielsen, Y.B. Pan,
J.H.vonWimmersperg-Toeller, W. Wiedenmann, J. Wu, Sau Lan Wu, X. Wu,
G.Zobernig
11
InstituteforParticlePhysics,ETH Honggerberg,8093Zurich,Switzerland.
1
AlsoatCERN,1211Geneva23,Switzerland.
2
NowatFermilab,POBox500,MS352,Batavia,IL60510,USA
3
Also at Dipartimento di Fisica di Catania and INFN Sezione di Catania, 95129
Catania,Italy.
4
NowatUniversityofFlorida,DepartmentofPhysics,Gainesville,Florida32611-8440,
USA
5
AlsoIstitutodiCosmo-GeosicadelC.N.R., Torino,Italy.
6
AlsoatGrouped'AstroparticulesdeMontpellier,UniversitedeMontpellierII,34095,
Montpellier,France.
7
SupportedbyCICYT,Spain.
8
SupportedbytheNationalScience FoundationofChina.
9
SupportedbytheDanishNaturalScienceResearchCouncil.
10
SupportedbytheUKParticlePhysicsandAstronomyResearchCouncil.
11
SupportedbytheUS DepartmentofEnergy,grantDE-FG0295-ER40896.
12
Nowat DepartementdePhysiqueCorpusculaire,UniversitedeGeneve,1211Geneve
4,Switzerland.
13
Alsoat DepartmentofPhysics,TsinghuaUniversity, Beijing,ThePeople's Republic
ofChina.
14
SupportedbytheLeverhulmeTrust.
15
Permanentaddress: UniversitatdeBarcelona,08208Barcelona,Spain.
16
SupportedbyBundesministeriumfurBildung undForschung,Germany.
17
SupportedbytheDirection desSciencesdelaMatiere,C.E.A.
18
SupportedbytheAustrianMinistryforScienceandTransport.
19
NowatSAPAG,69185Walldorf,Germany
20
NowatGrouped'AstroparticulesdeMontpellier,UniversitedeMontpellierII,34095
Montpellier,France.
21
NowatBNP Paribas,60325FrankfurtamMainz, Germany
22
SupportedbytheUS DepartmentofEnergy,grantDE-FG03-92ER40689.
23
Now at Institut Inter-universitaire des hautes Energies (IIHE), CP 230, Universite
Libre deBruxelles, 1050Bruxelles,Belgique
24
AlsoatDipartimentodiFisicaeTecnologieRelative,UniversitadiPalermo,Palermo,
Italy.
25
Deceased.
26
NowatSLAC,Stanford,CA94309,U.S.A
27
Nowat INFN Sezione di RomaII, Dipartimento di Fisica, Universitadi RomaTor
Vergata,00133Roma,Italy.
28
ResearchFellowoftheBelgiumFNRS
29
ResearchAssociateoftheBelgiumFNRS
30
NowatLiverpoolUniversity,LiverpoolL697ZE,UnitedKingdom
31
SupportedbytheFederalOÆceforScientic,TechnicalandCulturalAairsthrough
The study of exclusive meson and baryon pair production has long been
recognisedasapotentiallyimportanttestinggroundofQCD[1,2,3,4]. The
calculation factorizes into a hard scattering amplitude which at suÆciently
large momentum transfers is calculable in QCD, and a hadron distribution
amplitude
M
(x;Q). As this distribution amplitude is expected to be
process independent, measurements from many dierent reactions will be
complementary. Measurements in photon photon collisions are important
as they are the simplest calculable large-angle exclusive hadronic scattering
reactions.
In [2] it is shown that the leading order QCD result can be expressed
approximatelyinterms ofthecross sectionforthe twophotonproductionof
muon pairs:
d
dcos
( !M
+
M ) 4jF
M (W
2
)j
2
1 cos 4
d
dcos
( !
+
) (1)
Theanglecos
isthe scatteringangleofoneofthe twochargedparticles
Mwithrespecttothedirection,calculatedinthecenterofmassframe.
TheoverallangulardependenceofEq.1issin 4
whenonefoldsinthemuon
paircrosssection. ThemesonformfactorF
M
dependsonthestrongcoupling
constant
s
,theinvariantmassW
,andthepseudoscalardecayconstant
f
M .
This letter describes a measurement of the exclusive pion and kaon
pair production in two photon collisions using the data collected by the
ALEPH experiment at LEP. The absolute rate and the shape of the cos
distributions are compared to QCD calculations and previous experiments
at lower energies.
2 ALEPH Detector
The ALEPH detector has been described in detail elsewhere [5]. Critical
to this analysis is the ability to accurately measure and identify charged
particles. These are detected in a large time projection chamber (TPC)
supplemented by informationfrom the inner tracking chamber(ITC) which
is a cylindricaldriftchambersitting inside the TPC, and a two-layersilicon
strip vertex detector (VDET) which surrounds the beam pipe close to the
interaction point. These tracking detectors are placed inside a 1.5T axial
magnetic eld provided by a superconducting solenoidal coil. Charged
particle transverse momenta are measured with a resolution of Æp
t
=p
t
=
6 10 p
t
0:005 (p
t
in GeV =c). In addition to measuring momenta,
the TPC provides dE=dxinformation which allows particleidentication to
be performed. Surrounding the solenoid and TPC is the electromagnetic
calorimeter (ECAL) whose primary purpose is the identication and
measurementofelectromagneticclustersproducedbyphotons andelectrons.
It is alead/proportional-tubesamplingcalorimeter segmented in0:9 Æ
0:9 Æ
projectivetowers read outinthreesectionsindepth. It hasatotal thickness
of22radiationlengthsandarelativeenergyresolutionof0:18=
p
E+0:009,(E
inGeV).Inthis analysisitismainlyusedasaveto againsteventscontaining
electrons or photons which are readily identied by the longitudinal and
transverse structure of theirshowers. Outside theECAL liesthe ironreturn
yoke for the magnetwhich is instrumented with 23 layers of streamer tubes
to form the hadron calorimeter (HCAL). It has a relative energy resolution
for hadrons of 0:85=
p
E (E in GeV). The outermost detectors of ALEPH
are themuon chambers whichconsistoftwodouble-layers ofstreamer tubes.
The HCAL and muon chamber information is combined to identify muons
which are a major potentialbackground inthis analysis. Close tothe beam
pipe ataposition3meither sideof the interactionpointare two luminosity
calorimeters, the LCAL and SiCAL, which are electromagnetic calorimeters
specically designed to measure the luminosity via Bhabha scattering. In
this analysis they are used to veto events which contain signicant energy
deposits at smallangles.
The information from the tracking detectors and the calorimeters are
combined in an energy-ow algorithm [5]. For each event, the algorithm
provides a set of chargedand neutralreconstructed particles, called energy-
ow objects.
3 Selection of Exclusively Produced Pairs of
Charged Particles
Thedataforthisanalysisareselectedfrom837:5pb 1
collectedbetween1992
and 2000. During this period the LEP centre of mass energy ranged from
88 to 209 GeV. The initialselection obtains events with two charged tracks
exclusively produced by the process. Events were required to have:
two charged tracks, each of which has jcosj < 0:93, p
t
> 1:0GeV=c,
at least 5TPC hits and atleast 6 ITC hits;
areconstructedvertex inacylinderwithitsaxis8cmeitherside ofthe
interaction pointalong the beam directionand with aradius of 3cm;
p
t
<0:1GeV=c;
no otherenergy owobjectsapart from the two charged tracks;
invariant mass of charged tracks (assuming pion mass) between 2 and
50 GeV=c 2
;
the back-to-back triggerset, asdescribed insection 5.
4 Selection of and K Pairs
The purpose of this stepis toselectaspure asampleaspossible ofpion and
kaonpairs. First,any event containingatrack identied asanelectron oras
a muon was rejected. Electrons were identied using the standard ALEPH
method[5]. Formuons the standard identicationwas supplemented by the
procedure described in [6]. This allows the sensitivity for muon pairs to be
extended down tolowerinvariantmasses. Muons with momentum less than
3.0 GeV =c wererequired tohavedepositedenergy intheECALequaltoless
than half their momentum, and to havered more than 25% of the number
of HCAL planes expected for a muon. If the muon's momentum is greater
than 1.5 GeV=c then the energy deposited in the HCAL must be greater
than 80 % of its momentum, otherwise it must be greater than 60% of its
momentum.
The dE=dxinformation for each track was employed to give the
probability for each track to be a pion, kaon or proton. The product of
these gives the probability for the pair of tracks to both be pions, kaons or
protons. Theseare denoted as P (
+
; )
,P (K
+
;K )
and P (p;p)
respectively.
A mesonpair of type a was dened to have
P (a
+
;a )
>0:05
P (b
+
;b )
<0:05
where a = , b = (K;p) for pion pair selection, and a = K; b = (;p)
for kaon pair selection. For the kaon pair selection the invariant mass was
recalculatedusing thekaonmass foreachchargedparticle. EventswithW
greater than 6:0 GeV =c 2
were then rejected.
Afterstudyinganumberofbackgroundsources, asdescribed insection6,
the followingadditionalcuts were imposed. At leastone of the tracksin the
event must have a momentum such that the ionisation expected for a kaon
for a minimum ionising particle. The following cuts were applied only to
the pion sample, which isparticularlysusceptible tobackgroundsfromboth
muon pairs and non exclusive hadronproduction:
p
t
<0:05GeV=c ;
the angle between the tracks measured in the plane perpendicular to
the beam directionmust begreater than 3.13 radians;
if a track has momentum less than 1.5 GeV =c, then any associated
cluster in the electromagnetic calorimeter must have energy equal to
less than 50% of its momentum, and any associated cluster in the
hadron calorimeter must have energy equal to less than 10% of its
momentum.
The nal selectionconsisted of 165 kaon and 318 pion pair events.
5 Trigger EÆciency
The event selection described above was designed to select events in a
region where the ALEPH trigger is known to be highly eÆcient. This
analysistakesadvantageof aspecicALEPHtriggerwhichisredwhentwo
approximately back to back tracks are seen in the tracking detectors. The
eÆciency of this trigger was measured using the process ! e +
e which
is also independently triggered by energy deposits in the electromagnetic
calorimeter. From this the eÆciency curve was derived as a function of the
invariant mass of the nal state and used to correct the data. At invariant
massesof2:0GeV =c 2
theeÆciencywasmeasuredtobe95%,risingtoavalue
of 100% for invariant massesof 4:0GeV =c 2
and above.
6 Backgrounds
Background from ! +
was estimated using the program described
in [7], and that from ! +
using a program described in [8]. The
background from pairs was found to be negligible but muon pairs are a
signicant backgroundto the +
measurement.
The background to the pion pairs from misidentied kaon pairs was
computedusing thesimulatedkaonsignal(see section7). The normalisation
was chosen tomatchthe observed kaon signal. Anequivalentprocedurewas
used to nd the background to the kaon pairs from misidentied pions. As
eachother,theanalysiswasrepeateduntilastableresultwasobtained. This
was achieved afterveiterations.
The background from non exclusive !hadron production has been
checked using the PHOJET program, which is known to give a good
description of ALEPH untagged two photon physics events [9]. A small
background of events was found in the +
sample, almost entirely due
to events containingtwo pionsplus undetectedlowenergy photons. Overall
there wasan18%backgroundinthe pionsample andan11%background in
the kaon sample. The signicantbackgrounds are listed inTables 1and 2.
7 Calculation of Selection EÆciency
The selection eÆciencies for pion and kaon pairs were calculated using a
version of the program of [7], which simulates the QED process e +
e !
e +
e l +
l , modied in accordance with Eq. 1. The modication consisted
initially of weighting events by 1=W 4
. This set is referred to as `Simulation
A', and is shown in Fig 1. As can be seen the data sits slightly above the
curve sotwofurther sets, referred toassimulationsB and C were generated
with steeper dependence oncos
while stillbeing consistentwith the data.
Set B wasweightedby anadditional1=sin 4
term comparedtoset Awhile
set C was weighted by an additional1=sin 6
term. All three sets gave an
acceptable description of the data. Simulation B was used to obtain the
selection eÆciencies as it best described the data. The other two sets were
used in the calculation of systematicerrors as described below.
TheeÆcienciesareshown inTables3and4. Threequartersofeventsare
lostdue tothe boost of the nalstate, theremaininglossofeÆciency isdue
to the tightcuts needed to remove background events.
8 Systematic Uncertainties
The chief source of systematic error in this analysis is the dependence on
simulation to calculate selection eÆciencies and backgrounds. This can be
classied into two major components, rstly the dependence on accurate
simulation of the detector, and secondlya dependence on the physics of the
signal.
Anumberof criticalelementsof thedetector simulationwere studied. In
allcasesthiswasdonebyvaryingtheaspecttobestudiedintheMonteCarlo
sampleandrepeatingtheanalysis. ThedependenceonthedE=dxsimulation
